We received a report of a serious road traffic collision between two cars on the A4042 in Llanellen, near Abergavenny, at around 8.05pm on Thursday 2 January.
The collision involved a pink Peugeot and a blue Suzuki, with the Welsh Ambulance Service and the South Wales Fire and Rescue Service in attendance.
The rear seat passenger of the Suzuki, a 19-year-old female, was taken to Neville Hall Hospital in Abergavenny where she later died. She has been named as Rebecca Davies from Goytre.
A 52-year-old woman and a 63-year-old man, both from the Goytre area, sustained serious injuries and were taken to Neville Hall Hospital also, where they remain in a serious but stable condition.
The driver of the Peugeot, a 31-year-old from the Cardiff area, sustained serious injuries and was taken to the Neville Hall Hospital, where she remains in a critical condition.
Officers are requesting any motorists using the road around this time to please check any dash cam footage that may provide further information that will assist officers with their enquiries.
Anyone with any information or anyone who may have witnessed the incident is urged to contact Gwent Police on 101 or direct message us on Facebook or Twitter, quoting log 2000002327